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Paul Johnson 03-11-2011 03:42 PM

virtualbox versus kvm virtual machines. How account for video difference?
On a Lenovo Laptop with Ubuntu 10.10, I've installed the kvm/libvirt
as well as the VirtualBox virtual machines. Both install easily
enough, although I hasten to say that VirtualBox is much better at
noticing your hardware problems (it warned me BIOS had VM support off,
but virt-install did not) and its GUI has more flexible settings.
That difference is to be expected, since KVM is new and the GUI for it
is still young. I'd like to master KVM because I also administer a
lot of RedHat machines, so I explored that first on this laptop, and
after some frustration, I installed VirtualBox for comparison.

In those frameworks, I've installed Fedora 14, both the 32 and 64 bit
versions. The performance is about the same between the VirtualBox
and KVM machines, except for the following problem. The video display
in a KVM based VM is much much slower than the video display in the
VirtualBox. This is true whether or not the VirtualBox guest
additions are installed. The KVM machine's display updates much more
slowly, so if you open a terminal and compile something in there, the
display looks like, well, an old teletype machine. It reminds me
somewhat of what VNC used to look like in 1996 on Win95, the screen
slowly updating from top to bottom. The video refresh is so slow that
you can actually see programs like GQview draw their displays three
times when they start up. Know what I mean? GTK programs draw a blank
screen, then draw one layer of widgets, then another.

Inside KVM settings, there are options like "cirrus" or "vga" for
video cards, and one can also adjust the amount of VRAM. The default
install of the KVM machines only claimed a small video ram. But then
I increased the vram to 50meg, same as used by virtualbox. Still slow
video refresh.

If you have ideas about why this video speed difference might appear,
or settings I might try, please let me know. I'd like to know if this
is inherent in the two things, or if it is superficial.


Paul E. Johnson
Professor, Political Science
1541 Lilac Lane, Room 504
University of Kansas

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